The strontium isotope was analyzed by the combination system of laser-induced breakdown spectroscopy (LIBS) and molecular laser-induced fluorescence (MLIF). The fluorescence from A1Σ+ → X1Σ+ transition of strontium monoxide was observed by the selective excitation of individual vibrational band heads, and its spectral feature was investigated under atmospheric conditions being adjusted. In the fluorescence spectra of LIBS-MLIF, the signal intensity was improved about 10 times higher than the single LIBS, and each isotopologue could be clearly identified with an isotope shift of about 0.1 nm. By comparing these fluorescence signals, the isotope effect on molecular radicals in laser-induced plasma was investigated. The spatial distribution and the formation kinetics of isotopologues could be influenced by the isotopic effect, which exhibits a unique fluorescence behavior. It is noteworthy that the degree of isotopic effect on radicals could be modulated by controlling the atmosphere. Under the condition of reduced isotope effect, the strontium isotopic abundance could be accurately estimated within the error of 5% by the fluorescence ratio. The present work not only demonstrated the feasibility of LIBS-MLIF for the detection of strontium isotopes, but also provides novel insight into the molecular formation and isotopic effects in laser-induced plasma.